To change the fulfillment type of your Amazon listings, simply head over to your Amazon Listings Tab. Click on the listing you would like to edit and open the Edit Listing page.
Under "Listing Details" you will find the "Fulfilled By" field. Click on the dropdown to select either FBA or FBM. Now simply click "Save & Publish" and you're all done!
You can also change your fulfillment type in bulk from within your Amazon Listings Tab by selecting the listings you'd like to change and clicking on the "Bulk Edit" button at the top of the page. Select the fulfillment type and republish your listings.